Prosthesis with in-wall modulation

ABSTRACT

The wall of a prosthesis has a region which modulates communication through the porosity of the wall. The prosthesis is unitary, but may be assembled in successive bodies which are coalesced, so that the porous microstructure changes distinctly at stages through the thickness dimension of the wall. One embodiment is formed entirely of fluoropolymer, and has at least one surface adapted to support tissue regeneration and ingrowth. The modulation region is a stratum of high water entry pressure that reduces pulsatile hydraulic pressure transmission, or locally alters fluid-born-distribution of biological material through the wall and allows more natural gradients for tissue regeneration and growth in the outer region of the wall.

REFERENCE TO RELATED PATENTS AND PATENT APPLICATIONS

This application is a divisional of allowed U.S. application Ser. No.08/760,115, filed Dec. 3, 1996, now U.S. Pat. No. 5,824,050, andincorporated herein by reference.

This application relates to the commonly owned United States patentapplications having the following titles and patent application numbers:VASCULAR ENDOPROSTHESIS AND METHOD, patent application Ser. No.08/759,861; MULTI-STAGE PROSTHESIS, patent application Ser. No.08/760,113; EXPANDABLE SHIELDED VESSEL SUPPORT, patent application Ser.No. 08/759,877. It also relates to applicants' earlier U.S. Pat. No.5,433,909 and No. 5,474,824. The foregoing patents describe methods ofmaking extruded PTFE material having large oriented nodes, uniaxiallyoriented fibrils and a pore structure of oriented channels that differsat different surfaces, or that varies along the thickness dimension. Theaforesaid patent applications each describe constructions or methods ofuse for prostheses, which are further useful in the embodiments andapplications of the present invention. Each of the aforementioned UnitedStates patents and patent applications is hereby incorporated byreference herein in its entirety.

BACKGROUND

This invention relates to a lamellate polytetrafluoroethylene materialthat can be formed into an implant where there is an improvement in thesurgical handling accompanied with enhanced healing properties due tothe novel arrangement of variable porosity regions ofpolytetrafluoroethylene. This invention relates to materials utilized inthe production of devices for in vivo implantation, such as heart valveleaflets, sutures, vascular access devices or any related products, butmore particularly relates to vascular grafts, for example, to porouspolytetrafluoroethylene prostheses intended for placement orimplantation to supplement or replace a segment of a natural, biologicalblood vessel. It also relates to patches or supports for tissue repairor reinforcement. For simplicity of exposition below, the invention willbe discussed solely with relation to an implantable vascular graft, or aliner for a vessel which might, for example, be deliveredintraluminally.

Reference is made to commonly-owned U.S. Pat. Nos. 5,433,909 and5,474,824 which disclose a method and a product made thereby which is anextruded tube of polytetrafluoroethylene having a tailored porosity. Thetube is made by extrusion, of a perform having differently-prepared PTFEpaste at two or more levels along its radial section, followed bystretching and generally also sintering so as to achieve the desiredstrength and pore structure in the final product. As described in thosepatents and elsewhere, porosity may be tailored to achieve certaindesirable properties in the structure of nodes and fibrils that affectpermeability and various forms of tissue compatibility, such as thepromotion or prevention of tissue growth. In particular, the abovepatents describe a method of fabricating tubes of PTFE material havinggood mechanical strength, together with a combination of other featuresincluding one or more of a large reticulated node structure whichenhances tissue growth, a small pore structure which limits weeping ofthe graft, and different porosities through the thickness portion of thetube wall to achieve desired properties at both surfaces. The aforesaidcommonly-owned patents also describe a method of obtaining theseproperties in a single PTFE tube in which a property such as lubricationlevel has been consciously made non-uniform.

Other approaches to extruding a porous PTFE tube have involved stackingtwo preforms of different PTFE materials, or PTFE and a dissimilarmaterial, together and extruding a layered structure.

Still other approaches to incorporating PTFE as the sole or a largeportion of a vascular graft have involved numerous constructions. Theseinclude constructions wherein an inner tube is surrounded by one or moreother layers of tubing, foam or fiber wrapping to enhance its mechanicalcompliance and, for example, provide direct impermeability, or result inclotting which, after a short time, becomes impermeable. The inner tubeis generally formed of PTFE, selected for its highly advantageousbiocompatibility properties in the blood path. Various outer layers mayconsist of fibers either helically wound or electrostatically flocced,films of thin material, tape wrap generally also of thin material, orcoatings. Materials used for these layers may also include impermeablepolyurethane or other soluble polymer coatings, emulsions and also PTFEfilms.

These composite structures are in some ways similar to the earliergeneration of fabric grafts made of woven or knitted Dacron or the like,and each represents an attempt to address or optimize some of thevarious constraints encountered in trying to replace a vessel withmaterial which is strong, capable of long term patency and has somedegree of tissue compatibility.

In general, however, conventional vascular grafts manufactured fromporous polytetrafluoroethylene have limitations in surgical handling andhealing.

Presently, many vascular grafts exhibit some degree of weeping or bloodloss during implantation. A variety of factors effect this surgicalcomplication, one being prewetting of vascular grafts with heparinizedsaline or antibiotics to render the surface thrombus and infectionresistant. Prewetting of the graft results in a reduction of thehydrophobic properties with an effective increase in permeability.Cohesion of platelets and adhesion of fibrin in the graft wall caninitiate the coagulation cascade resulting in thrombus formation. Thethrombi are responsible for the formation of emboli in tubularprosthesis with small diameters.

Native arteries and veins have a common pattern of organization made upof three layers: an internal intima, surrounded by a media, and then anexternal adventitia. Each of these layers has a predominant structureand cell-type. The walls of arteries are built of elastin, collagen, anon-fibrous glucosaminoglycan-rich matrix and smooth muscle cells. Themicroscopic structure of the artery wall correlates with the function ofthe various wall-layers and components.

Several studies support the belief that there is a net transport ofmacromolecules across the arterial wall. The transport process iscontrolled by diffusion, convention, and forces. Convection isassociated with the hydraulic flux resulting from pressure or osmoticdifferences across the arterial wall. Diffusion occurs in response to aconcentration driving force.

While a number of vascular grafts, or processes for preparing the same,provide for a stronger graft, such grafts do not generally possess adifferential permeability effective to achieve enhanced healing andtissue ingrowth, and at the same time offer improved surgical handling.

There is a need for an in vivo implantable material device, and inparticular vascular grafts which are formed as a lamellate structurethat mimics the natural artery with differential cross-sectionpermeability composed of collagen and elastin and is acceptable to thesurrounding tissue.

It remains desirable to provide prostheses or material having enhancedtissue compatibility or long term patency or growth compatibilitycharacteristics.

SUMMARY OF THE INVENTION

This is achieved in accordance with the present invention by providingan implantable or prosthetic material with at least first, second andthird regions through the wall thickness extending continuously alongthe length and width thereof, and wherein material of the innermost andoutermost regions has a cellular compatibility property such as nodesize or reticulation structure, while at least one, preferably aninterior, region of the wall modulates hydraulic pressure otherwisepassing through the porosity of the prosthesis. The first, second andthird regions join or merge continuously together along their boundingsurfaces, and form a unitary wall.

The products comprising the instant invention have a very broadapplication in medical devices, such as vascular grafts, endovasculardevices, vascular access devices, transcutaneous access devices,synthetic heart valve leaflets, artificial organ implants, etc. In apreferred embodiment, each cross-section region of the implant can bedistinguished from other regions by having different pore size, poreshape and porosity. Indeed, the fibril-nodal microstructure throughoutthe matrix may have the internodal distance, i.e, pore size, in onesection at least two to twenty times that for its adjacent sections. Anin vivo material having three cross-section regions, for example, theinternodal distance of the pores of luminal surface of PTFE vasculargraft is about 20 or 30 microns with a corresponding WEP of 200 mm Hgand a specific node/fibril geometry. The internodal distance of thepoces of the next zone comprise a range from about 1 to about 10 micronswith a corresponding WEP of 400 mm Hg or greater and a specificnode/fibril geometry, preferably about 5 to 10 microns. The pore size isexcellent for cell growth mediator permeability, as distinguished, forexample from total impermeability which causes an undesirable state ofencapsulation. Another embodiment of the present invention includes aluminal, second, and third zone of material as previously describedwhereby the third zone has a pore size range of 50 to 500 microns with acorresponding WEP of 100 mm Hg or less and a specific node/fibrilgeometry, preferably with an internodal distance of about 50 to 100, toeffectively promote fibroblast tissue ingrowth, as the healing processprogresses. The lamellate structure of the present invention offers awall architecture similar in nature to that of a native vessel whichcontains an intima, media, and adventitia.

A further embodiment of the present invention includes in vivoimplantable material as described above in the form of a sheet, tube orenclosure comprising a luminal, a second and a third region aspreviously described. Another embodiment of the present inventionincludes the luminal, second and third region of material as previouslydescribed with the third region being filled to provide a sourcelocation for drug delivery.

For a vascular prosthesis, the outer wall may have a porosity or regularstructure of channels which is compatible with and serves as amicroscaffolding structure for the growth of connective tissue. Theinner face of the prosthesis on the other hand may have a smaller porestructure, optimized for attachment of a neointima for reconstituting anatural biological flow surface at the interior of the vessel. Themodulation region serves the function of blocking the direct orimmediate transmission of hydrostatic pressure or fluid migrationthrough the thickness dimension of the wall, and prevents through-growthof tissue, allowing a stratification of tissue layers to redevelop overtime in a more natural fashion after the prosthesis has been implanted.Pore structure of the modulation region may be irregular, and generallyis either small in size, or tortuous in path. The modulation region mayalso have non-existent porosity, i.e., be a continuous solid.

The prosthesis may be constructed from plural layers or tubes ofmaterial by radially nesting a first, second and third layer ofmaterial, either as tubes, wound sheets or a wrap and then coalescingthe three separate bodies together into a continuous wall body in whicheach region through the thickness retains the structure of the startingmaterial for that region. Preferably, the entire structure is made fromPTFE, or PTFE with another fluoropolymer.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will be understood from thefigures herein wherein:

FIG. 1 is a cross-sectional image through a wall of a first embodimentof the invention;

FIG. 2 is a cross-sectional image through a wall of a second embodimentof the invention;

FIG. 3 is a cross-sectional image through a wall of a third embodimentof the invention;

FIG. 4 is a cross-sectional image through a wall of a fourth embodimentof the invention;

FIG. 5 is a cross-sectional image through a wall of a fifth embodimentof the invention;

FIG. 6 is a cross-sectional image through a wall of a sixth embodimentof the invention;

FIG. 7 is a cross-sectional image through a wall of a seventh embodimentof the invention;

FIG. 8 is a cross-sectional image through a wall of a eighth embodimentof the invention;

FIG. 9 is a cross-sectional image through the wall of a ninth embodimentof the invention; and

FIG. 10 shows a vascular prosthesis according to any of the aboveembodiments of the present invention.

DETAILED DESCRIPTION

FIG. 10 illustrates an implantable prosthetic member 10 according to thepresent invention, which, is shown in the figure as a tubular member,suitable for implantation as a vascular graft. The member 10 has aninner wall 1 and an outer wall 2 with a thickness dimension extendingtherebetween. As further illustrated in FIG. 10, there are at leastthree continuous regions adjacent to each other and extending along theentire area of the member namely, regions a, b and c, illustrativelyshown in the Figure as concentric strata from the inside to the outside.As described in more detail below, the successive regions a, b, c arenot separate structures but are portions of the same wall, and aredistinguished by their structural properties as relates in particular toaspects of porosity.

In general, each embodiment of the invention includes at least oneregion having a zero or sufficiently low porosity that it effectivelyacts as a barrier to fluid penetration or a barrier which modulatestransmission of hydraulic pressure pulsation through the thin wall ofthe prosthesis. This barrier region may be a completely pore-freestratum, a stratum having small pore size, or a stratum having a highdensity of crossed, irregular, dead end, or closed cell pores such thatit carries out its modulation or barrier function. In the latter case,even large pore material may be used, but its water entry pressure (WEP)is high. This stratum may exist at the region of inner surface 1, theregion of outer surface 2, or an intermediate stratum as shown by theposition of region b in FIG. 10.

In material science, there is a distinction between material porosityand permeability. Porosity is a direct measure of the physical voidvolume contained with a boundary, whereas permeability refers to theaccessibility of that void volume. Permeability is usually expressed asa rate of flow of liquid or gas per unit area, as a function ofdifferential pressure.

In a porous, fibrous material, that part of the total porosity which isavailable to fluid flow is also called the "effective porosity." Thepressure required to force a liquid into a pore is a function of poresize and geometry, liquid surface tension, and solid/liquid contactangle. Surface tension opposes the entry of any nonwetting liquid into apore, and this opposition may be overcome by external pressure.

Expanded PTFE material is characterized by lengthwise-oriented fibrilsinterrupted by transverse nodes. The pore size in microns is typicallydetermined by measuring fiber length between the nodes (internodaldistance). To compute fibril length, the material is viewed undersufficient magnification. A fibril length is measured from one edge ofone node to the edge of an adjacent node. Fibril lengths are measuredfrom the sample to compute a mean fibril length.

Nodes and fibrils may be further characterized by their relativegeometry. That is, nodes by length, width, and height; and fibrils, bydiameter and length. It is the relative geometry of nodes to fibrils, aswell as, internodal distance that determines porosity and permeabilityof porous PTFE.

Permeability to fluid flow can be determined by measuring the amount ofpressure required for water to permeate the pores of the material. Tocompute water entry pressure (WEP) one subjects the material to anincrementally increasing water pressure until small beads of waterappear on the surface. WEP is a gage which can be used to equateporosity to permeability.

Vascular graft porosity is a measure of the void fraction within theprosthesis wall and is believed to give a rough prediction of thecapacity of the graft to anchor newly formed surrounding tissue afterimplantation, whereas permeability is associated with fluid flow throughthe graft wall.

Vascular permeability or hydraulic conductivity is related to materialporosity. WEP is a good measuring technique to assess this trait becauseit closely mimics the permeation process at the blood/prosthesisinterface. WEP is defined as the pressure value necessary to push waterinto the pores of a synthetic tubular substrate and can be classifiedas: High (>400 mm Hg), Medium (200-400 mm Hg), and Low (<200 mm Hg).

It has been widely accepted since the nineteenth century that thehydrostatic pressure difference across the arterial wall is capable oftransporting water from the blood into the surrounding interstitialspace. The view has long been held that a continuous transport ofmaterial occurs across the arterial wall, from its inner to its outersurface. Solutes flow past the endothelium gradually passing through thevarious arterial wall layers eventually being transferred to thelymphatics or adventitia.

The filtration coefficients of the wall are dependent on the hydraulicconductivity of both the intima and media. The artery wall is aheterogeneous porous medium in which interstitial fluid can flow throughthe interstices between cells and tissue mimicking a semipermeablemembrane with hydrostatic and osmotic pressure components. The osmoticpressure difference across the vessel wall is assumed to be smallcompared with the hydrostatic pressure or hydraulic conductivity.

More controlled healing and tissue ingrowth is achieved by providing aspecific region (outer) for cell penetration, followed by a region(barrier) that does not allow free cellular penetration/permeation butinstead, allows the transport of plasma solutes such as cellularmediators (proteins, growth factors, etc.) This barrier minimizes therelatively large hydraulic force present in arterial transport thatretards tissue ingrowth. Reports have shown that a negative pressureexists within the perigraft space, while blood components (cells,particles, etc.) are isolated to the blood side of the device.

A vascular graft formed from the lamellate structure of the inventionmimics the natural artery with a cross-section that offers differentialpermeability properties resulting in a healing response acceptable tothe surrounding tissue.

In a prototype embodiment of the invention, a prosthesis 10 as describedabove was fabricated in a multi-step procedure by assembling threephysically separate bodies of material together in successive strata andthen joining or coalescing them into a single unit.

When a vascular prosthesis is fabricated according to this method,preferably at least one of the bodies is a tube which may, for example,be an axially-stretched tube having a porous structure of internodalspace oriented transverse to its surface. Advantageously, the nodalspacing, orientation or structure of successive strata may be offset,non-matching or misaligned to introduce or enhance a barrier orhydraulic modulation effect. For example, a prosthesis may be formed byplacing a first PTFE tube on a mandrel, wrapping a ribbon of PTFE in anoverlapped or non-overlapped spiral winding over the tube outer surface,and then placing another PTFE tube over the assembly. For thisconstruction, the outermost tube has preferably been previously radiallyexpanded. Heat is then applied to the assembly, optionally with a radialcompressive force, to shrink back the outer tube and coalesce the threeseparate bodies together into a unitary prosthesis. Although effectively"welded" together, there are no visible deformations, and thethrough-wall properties change abruptly at the interface of each stratumor region with the next.

EXAMPLE 1

PTFE resin (Fluon CD-123 obtained from ICI Americas) was blended with100 grams of "Isopar H odorless solvent (produced by Exxon Corporation)per pound of PTFE, compressed into a preform billet and extruded into a3.5 mm I.D. and 4.0 mm O.D. tube in a ram extruder having a reductionratio of about 200:1 in cross-sectional area from billet to extrudedtube. After removal of lubricant, the extruded tube was expanded andsintered, according to the method described in U.S. Pat. Nos. 5,433,909and 5,474,824, which patents are hereby incorporated by reference hereinin their entirety, under various conditions to produce material withdifferent hydraulic porosities. This produced three different tubes,denoted A, B and C, which were used as starting materials for theconstructions described below.

Stretch conditions and resultant hydraulic porosities are given below inTable 1.

                  TABLE 1                                                         ______________________________________                                        Expansion              Hydraulic Porosity                                     Temp (° C.)                                                                       Rate (in/sec)                                                                             Ratio (%)                                                                              WEP (mm Hg)                                   ______________________________________                                        (A)  320       .004        3:1    100                                         (B)  300       .018        3:1    200                                         (C)  250       7.5         2.5:1  600                                         ______________________________________                                    

Material (B) was radially expanded to a 4 mm ID on a stainless steelforming mandrel, covered with material C that had been previouslydilated to a 5 mm ID, restrained to prevent longitudinal shrinkage, andtransferred to an oven at 360° C. for 5 minutes, to prepare a primarylamellate. The primary lamellate was removed from the oven and allowedto cool, covered with material (A) that had been previously dilated to a5 mm ID, restrained to prevent longitudinal shrinkage, and placed in anoven at 360° C. for 10 minutes, to prepare a final lamellate structure(material B/C/A), a cross-section of which is shown in FIG. 1.

Material (B) was radially expanded to a 4 mm ID on a stainless steelforming mandrel, covered with material (A) that had been previouslydilated to a 5 mm ID, restrained to prevent longitudinal shrinkage, andtransferred to an oven at 360° C. for 5 minutes, to prepare a primarylamellate. The primary lamellate was removed from the oven and allowedto cool, covered with material C that had been previously dilated to a 5mm ID, restrained to prevent longitudinal shrinkage, and placed in anoven at 360° C. for 10 minutes, to prepare a final lamellate structure(material B/A/C), a cross-section of which is shown in FIG. 2.

Material C was radially expanded to a 4 mm ID on a stainless steelforming mandrel, covered with material (B) that had been previouslydilated to a 5 mm ID, restrained to prevent longitudinal shrinkage, andtransferred to an oven at 360° C. for 5 minutes, to prepare a primarylamellate. The primary lamellate was removed from the oven and allowedto cool, covered with material (A) that had been previously dilated to a5 mm ID, restrained to prevent longitudinal shrinkage, and placed in anoven at 360° C. for 10 minutes, to prepare a final lamellate structure(material C/B/A) a cross-section of which is shown in FIG. 3.

EXAMPLE 2

Material (B) was radially expanded to a 4 mm ID on a stainless steelforming mandrel, biaxially wound with commercially available PTFE ribbonon a helix winding apparatus, and covered with material (A) that hadbeen previously dilated to a 5 mm ID, restrained to prevent longitudinalshrinkage and placed in an oven at 360° C. for 10 minutes to prepare alamellate structure (Material B/Biaxial wrap/Material A), across-section of which is shown in FIG. 4.

PTFE ribbon was biaxially wound onto a stainless steel forming mandrel,covered with material (B) that had been previously dilated to a 5 mm ID,restrained to prevent longitudinal shrinkage and placed in an oven at360° C. for 5 minutes, to prepare a primary lamellate. The primarylamellate was removed from the oven and allowed to cool, covered withmaterial (A) that had been previously dilated to a 5 mm ID, restrainedto prevent longitudinal shrinkage and placed in an oven at 360° C. for10 minutes to prepare a final lamellate structure (Biaxialribbon/material B/material A), a cross-section of which is shown in FIG.5.

Material (B) was radially expanded to a 4 mm ID on a stainless steelforming mandrel, covered with material (A) that had been previouslydilated to a 5 mm ID, restrained to prevent longitudinal shrinkage, andplaced in an oven at 360° C. for 5 minutes, to prepare a primarylamellate. The primary lamellate was removed from the oven and allowedto cool, covered with a biaxial wrap of PTFE ribbon, restrained toprevent longitudinal shrinkage and placed in an oven at 360° C. for 10minutes to prepare the final lamellate structure (Material B/MaterialA/Biaxial ribbon), a cross-section of which is shown in FIG. 6.

EXAMPLE 3

Material (B) was radially expanded to a 4 mm ID on a stainless steelforming mandrel, longitudinally wrapped with commercially available PTFEribbon, and covered with material (A) that had been previously dilatedto a 5 mm ID, restrained to prevent longitudinal shrinkage, and placedin an oven at 360° C. for 10 minutes to prepare a lamellate structure(Material B/Longitudinal wrap/Material A), a cross-section of which isshown in FIG. 7.

PTFE ribbon was placed longitudinally around a stainless steel mandrel,covered with material (B) that had been previously dilated to a 5 mm ID,restrained to prevent longitudinal shrinkage, and placed in an oven at360° C. for 5 minutes, to prepare a primary lamellate. The primarylamellate was removed from the oven and allowed to cool, covered withmaterial (A) that had been previously dilated to a 5 mm ID, restrainedto prevent longitudinal shrinkage and placed in an oven at 360° C. for10 minutes to prepare a final lamellate structure (Longitudinalribbon/Material B/Material A), a cross-section of which is shown in FIG.8.

Material (B) was radially expanded to a 4 mm ID on a stainless steelforming mandrel, covered with material (A) that had been previouslydilated to a 5 mm ID, restrained to prevent longitudinal shrinkage, andplaced in an oven at 360° C. for 5 minutes, to prepare a primarylamellate. The primary lamellate was removed from the oven and allowedto cool, covered with a longitudinal wrap of PTFE ribbon, restrained toprevent longitudinal shrinkage and placed in an oven at 360° C. for 10minutes to prepare a final lamellate structure (Material B/MaterialA/Longitudinal ribbon), a cross-section of which is shown in FIG. 9.

To assess the in vivo performance of prostheses prepared in thisfashion, four millimeter lamellate grafts of various configurations wereimplanted into the carotid and/or femoral arteries of dogs. Explantswere taken at 14, 30, 60, and 180 days. The presence of an intrawall lowporosity, high WEP region produced enhanced tissue ingrowth compared tomaterial without such a region, leading applicant to believe thathydraulic forces play a role in the healing process of implantabledevices.

The foregoing examples describe the preparation of a tube havingenhanced properties as a vascular graft or tissue patch in which aregion of high WEP in the wall modulates communication through the walland the biological response and growth processes occurring in orcontiguous to the wall. As such, it provides an improved constructionapplicable to a broad range of implant and surgical protheses. Theinvention being thus disclosed, variations and modifications will occurto those skilled in the art and are considered to be within the scope ofthe invention and its equivalents, as defined by the claims appendedhereto.

We claim:
 1. A method of making a device, such method comprising thesteps ofplacing at least first and second porous PTFE portions one overthe other with a low porosity PTFE portion therebetween to form anadhesive-free assemblage, and heating the assemblage to coalesce theassemblage into a unitized device.
 2. The method of claim 1, wherein thedevice is a prosthesis and said first and second portions are tubes. 3.The method of claim 2, wherein said substantially non-porous PTFEportion is a PTFE tube, and the step of placing to form an assemblageincludes the steps of placing said tubes on a mandrel, and wherein thestep of heating includes heating the assemblage while it is on themandrel so that force of shrinkage of the tubes against the mandrel whenheated joins the tubes together without solvent.
 4. The method of claim1, wherein the low porosity PTFE portion has a bias orientation and athickness below approximately one millimeter.
 5. A method of forming adevice, such method comprising the steps ofproviding a first portion ofporous PTFE, providing a second portion of low or non-porous materialsurrounding said first portion, providing a third portion of porous PTFEsurrounding said second portion, and coalescing said first portion, saidsecond portion and said third portion into a single wall wherein saidsecond portion modulates cell growth in portions of said wall formed bysaid first and third PTFE portions and limits through penetration ofcell growth.
 6. A method of making a prosthetic device, the methodcomprising the steps ofproviding first and third tubes having acellularly penetrable structure, providing a second permeable tubebetween said first and third portion having a structure effective tolimit cell growth between said first and third portions, and coalescingsaid first tube, said second tube, and said third tube into a unitaryprosthesis.
 7. The method of claim 6, wherein the step of providing saidfirst and third tubes includes the step of forming said first tube froma fluoropolymer material.
 8. The method of claim 6, wherein the step ofproviding said second permeable tube includes the step of forming saidsecond tube from a fluoropolymer material.
 9. The method of claim 6,wherein the step of coalescing includes the step of applying heat tosaid first, second, and third tubes.
 10. The method of claim 9, whereinsaid step of coalescing further includes the step of applying a radialcompressive force to said third tube.
 11. The method of claim 6, furtherincluding the step of radially expanding said third tube prior to thestep of providing said second tube between said first and third tubes.12. A method of making a prosthetic device, the method comprising thesteps ofproviding a first fluoropolymer tube, providing a permeablefluoropolymer material having a permeability effective for modulatinghydrostatic pressure, wrapping said fluoropolymer material over theouter surface of the first tube, placing a second fluoropolymer tubeover said fluoropolymer material and said first tube, and coalescingsaid first and second tubes and said fluoropolymer material to form aunitary prosthesis.
 13. The method of claim 12, wherein the step ofcoalescing includes the step of applying heat to said first and secondtubes and said fluoropolymer material.
 14. The method of claim 13,wherein said step of coalescing further includes the step of applying aradial compressive force to said second tube.
 15. The method of claim12, further including the step of radially expanding said second tubeprior to the step of placing said second tube over said fluoropolymermaterial and said first tube.
 16. The method of claim 12, wherein thestep of wrapping includes the step of wrapping said fluoropolymermaterial in spiral winding over the outer surface of said first tube.17. A prosthetic member comprisinga prosthesis wall having first,second, and third coaxial, permeable portions, said first and thirdportions having a cellularly penetrable structure, and said secondportion positioned between said first and third portions and having apermeability less than said first portion to modulate cellularpenetration of said first and third portions.
 18. The prosthetic memberof claim 17, wherein said second portion has a porosity less than saidfirst portion.
 19. The prosthetic member of claim 17, wherein said firstand third portions are continuous tubes and said second portion is abiased wrap.
 20. The prosthetic member of claim 17, wherein said firstand third portions are continuous tubes and said second portion is anon-biased wrap.
 21. The prosthetic member of claim 17, wherein saidsecond portion is permeable to plasma solutes.
 22. The prosthetic memberof claim 17, wherein said first, second, and third portions are formedof a fluoropolymer.
 23. The prosthetic member of claim 17, wherein saidfirst, second, and third portions are coalesced together to form saidprosthesis wall.
 24. The prosthetic member of claim 17, wherein saidsecond portion has a water entry pressure greater than said firstportion.
 25. The prosthetic member of claim 17, wherein said secondportion has a permeability effective for modulating hydrostaticpressure.
 26. The prosthetic member of claim 17, wherein said secondportion has a permeability effective to allow passage of gas and fluidat a controlled rate between said first and third portions.
 27. Aprosthetic member comprisinga prosthesis wall having first, second, andthird coaxial, permeable portions, said first and third portions havinga cellularly penetrable structure, and said second portion positionedbetween said first and third portions and having a structure effectiveto limit cell growth between said first and third portions.
 28. Theprosthetic member of claim 27, wherein said second portion is permeableto plasma solutes.